1. Field of the Invention
The present invention relates to a rotation angle detecting device using a GMR device which indicates great change in resistance according to change in external magnetic fields.
2. Description of the Related Art
A conventional rotation angle detecting device is illustrated in FIGS. 4 and 5, with FIG. 4 being a plan view illustrating the principal portions of a conventional rotation angle detecting device, and FIG. 5 a circuit configuration diagram of a magnetism detecting unit. The rotation angle detecting device shown in FIG. 4 is a rotation angle detecting device capable of two outputs for the rotation angle output thereof.
As shown in FIG. 4, with a conventional rotation angle detecting device 1, a board (unshown) is divided into four block (regions) A, B, C, and D, by perpendicularly-intersecting imaginary axial lines for the X axis and Y axis. A pair of Giant Magneto-Resistance devices (hereafter referred to as “GMR devices”) G1 and G2 are disposed in the block A, a pair of GMR devices G3 and G4 are disposed in the block B, a pair of GMR devices G5 and G6 are disposed in the block C, and a pair of GMR devices G7 and G8 are disposed in the block D.
A disc-shaped or ring-shaped rotating member 2 is provided facing the four blocks A, B, C, and D, so as to be capable of rotating in the α1 and α2 directions on the point of origin O of the X axis and Y axis.
Magnets M1 and M2 are provided on the perimeter of the rotating member 2 at positions symmetrical with regard to the point of origin O. The one magnet M1 has the inner circumferential side thereof magnetized to the N pole, and the other magnet M2 has the inner circumferential side thereof magnetized to the S pole. Accordingly, a magnetic field heading from the N pole of the magnet M1 to the S pole of the magnet M2 is generated by the rotating member 2, with the GMR devices G1 through G8 of the blocks A through D situated within this magnetic field.
FIGS. 4 and 5 shows the direction of the fixed magnetism of the fixed magnetic layers of the GMR devices with arrows e. As shown in FIG. 4, the direction e of the fixed magnetism is the same within each pair of GMR devices. Also, the direction e of the fixed magnetism is the same for the A and B blocks, and the same for the C and D blocks, but opposite between the A and B blocks and the C and D blocks.
The GMR devices G1 through G8 form two Wheatstone bridge circuits by being connected as shown in FIG. 5. The first Wheatstone bridge circuit is configured of the GMR devices G1 and G2 of the block A and the GMR devices G5 and G6 of the block C, and the second Wheatstone bridge circuit is configured of the GMR devices G3 and G4 of the block B and the GMR devices G7 and G8 of the block D.
With the first Wheatstone bridge circuit, a predetermined electric power source voltage V is applied to the connection between the GMR devices G1 and G5, and the connection between the GMR devices G2 and G6 is grounded. The connection between the GMR devices G1 and G6, and the connection between the GMR devices G2 and G5, are output terminals T1 and T2.
In the same way, with the second Wheatstone bridge circuit, a predetermined electric power source voltage V is applied to the connection between the GMR devices G3 and G7, and the connection between the GMR devices G8 and G4 is grounded. The connection between the GMR devices G3 and G8, and the connection between the GMR devices G7 and G4, are output terminals T3 and T4.
Upon the rotating member 2 being rotated in the α1 and α2 directions, the direction of the magnetic field changes according to the rotation angle. In the event that the magnetic field rotates within a plane parallel to the board on which the GMR devices G1 through G8 are formed, the direction of magnetization of the free magnetic layer (not shown) provided within the GMR devices G1 through G8 changes according to the direction of the rotating magnets M1 and M2. Consequently, the resistances R1 through R8 of the GMR devices G1 through G8 cyclically change with regard to the rotation angle of the rotating member 2.
The direction of the fixed magnetism is opposite for the GMR devices G1 and G6, the GMR devices G2 and G5, the GMR devices G3 and G8, and the GMR devices G4 and G7, so the change due to rotation of the magnets is opposite between the resistance values R1 and R6, the resistance values R2 and R5, the resistance values R3 and R8, and the resistance values R4 and R7.
Accordingly, the differential output between the output terminals T1 and T2 of the first block circuit, and the differential output between the output terminals T3 and T4 of the second block circuit, can both be obtained as sin θ functions, wherein θ is a variable representing the rotation angle of the rotating member 2.
However, with the above-described conventional rotation angle detecting device 1, the GMR devices are arrayed in sets of pairs of GMR devices arrayed in parallel, so in the event of configuring Wheatstone bridge circuits by combining the GMR devices as shown in FIG. 5, with the first block circuit, the center point P1 of the GMR device G1 and the GMR device G6, and the center point P2 of the GMR device G2 and the GMR device G5, do not agree with the point of origin O. In the same way, with the second block circuit, the center point of the GMR device G3 and the GMR device G8, and the center point of the GMR device G4 and the GMR device G7, do not agree with the point of origin O.
This means that magnetic field of the same intensity and direction cannot be applied from the magnets M1 and M2 to the GMR device G1 and the GMR device G6. In the same way, a magnetic field is applied to the GMR device G2 and the GMR device G5, the GMR device G3 and the GMR device G8, and the GMR device G4 and the GMR device G7, with different intensity and direction.
Accordingly, the amount of change as to the rotation angle θ is not the same for the resistances R1 through R8, leading to deterioration in the sine waveform which is the differential output detecting from the first Wheatstone bridge circuit, and in the sine waveform which is the differential output detecting from the second Wheatstone bridge circuit, which has been a problem since the detection accuracy of the rotation detecting device 1 accordingly deteriorates.